Guild halls are boring ... Let us pillage them!

This is what I'm thinking: Have a special room you can acquire for your guild. This room will basically allow you to access a combat zone that you can place structures in. This room will have a special elevator or elevators in it which you have to invest either crystals or energy to activate, but whose destination you are able to choose, like the combat zones of other guilds. There might also be neutral territories ripe for conquering! Powering those elevators will take some time, however, and a guild will get a warning when their combat zone is about to be breached. A guild may be able to either delay or advance the arrival of the elevator, so that the intrusion may arrive at a more appropriate time zone.

You won't have to build a combat zone for your guild, but building it will allow you to lower the guild upkeep cost, as long as you maintain the combat zone active. If a guild attacks your combat zone and defeats you, you will lose access to it and be sent back to your guild hall, as well as quite possibly the defensive structures you had built in your combat zone. You will not obtain any benefits from it during the time it is disabled. All members will get an alert when a combat zone is being breached, and when the breach is about to occur, but only a few, let's say about 6, will be able to participate. You may also choose to disable your combat zone before an invasion reaches your own

Taking over another guild's combat zone will provide you with increased combat room effectiveness, but while you are taking a guild's combat zone, your own may also come under attack. Furthermore, if there are neutral territories, since these are shared, some of these may come under attack themselves. During each intrusion, only a fixed number of players may participate per activate elevator, and only to their alloted maximum of, say 6 members, so multiple elevators may need to be activated to effectively conquer the combat zone.

You will be able to employ a variety of defenses. Rocket turrets will be the most expensive, but you will also be able to employ the help of your gremlin allies and the local wildlife, though you may need to place the prerequesite structures in your combat zone or need to have taken over the territory that would support it. There would be a build limit, but it will be based solely on the amount of territory and the type of territory taken ... and you may still need to use up energy in the process.

Combat zones will also support alliances, so small guilds will be able to survive by making alliances with other guilds.